The refractory metal chosen for ITER’s divertor and considered for next-generation reactor first walls — withstanding the most extreme heat fluxes and particle bombardment in any engineering application.
Tungsten (W, atomic number 74) has the highest melting point of any element (3422°C), extremely low sputtering yield under plasma bombardment, low tritium retention, and does not produce long-lived radioactive isotopes under neutron irradiation. These properties make it the material of choice for plasma-facing components in fusion reactors, particularly in the divertor where heat fluxes reach 10–20 MW/m² — comparable to the surface of the Sun.[1]
Brittleness: Tungsten is brittle below its ductile-to-brittle transition temperature (DBTT, ~400°C for unirradiated W, higher after neutron damage). This makes it prone to cracking under thermal cycling. Neutron damage: 14 MeV fusion neutrons create displacement damage and transmutation products (rhenium, osmium) that further embrittle the material. Recrystallization: At temperatures above ~1200°C, tungsten recrystallizes, becoming even more brittle.[2]
Research focuses on: tungsten alloys (W-Re, W-Ta) with improved ductility; tungsten fibre-reinforced composites (Wf/W) inspired by ceramic matrix composites; powder-metallurgy tungsten with controlled microstructure; and self-passivating tungsten alloys (W-Cr-Y) that form protective oxide layers in case of air ingress accidents.[3]